In the related art, a transmission system adapted to modulate and transmit light based on a differential phase shift modulation method has been developed. In the differential phase shift modulation method, bits of information are allocated to the phase difference of waves which are subsequent or precedent by only a single period of the carrier wave (light). Therefore, in order to demodulate the transmitted differential phase shift keying-modulated optical signals with high accuracy, it is necessary for the receiver side to demodulate the optical signal after delaying it accurately by a single period. A technique of delaying the phase difference of the optical signal accurately by a single period during demodulation is disclosed in, for example, Patent Documents 1 and 2.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-080304
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-032514